In total ankle replacement, the degenerated articular surfaces are removed and replaced with an artificial joint called a prosthesis. The goals are a) to relieve the pain, b) to restore the original mobility, c) to restore the original stability of the joint.
Human joint replacement design has to address the following traditional dilemma. In an articulation subjected to multidirectional loads like the ankle, an unconstrained or semi-constrained type of prosthesis that allows for the necessary axial and transverse as well as flexion/extension mobility requires incongruent contact, which however leads to inadequate load-bearing capacity. Conversely, a congruent type of prosthesis produces undesirable constraining forces that overload the anchoring system. The use of a floating bearing element could be adopted in some cases to solve the dilemma. Meniscal bearing prostheses provide for complete congruence over the entire range of positions of the joint with minimally constrained components to enable the soft tissues still to control the physiological motion of the joint. However, potential problems pertaining to the risk of subluxation and/or dislocation can be envisioned. This depends respectively on the degree of distracting stability of the joint and the degree of entrapment of the bearing element in between the bone-anchored components.
Therapy resistant ankle pain and the disadvantages of ankle arthrodesis (i.e. fusion of the two damaged surfaces) for the ankle led to the development of numerous ankle joint prostheses since the early Seventies. After the first encouraging results, ankle arthroplasty acquired a bad reputation based on many long-term follow-up clinical-radiographic studies. The frequent failures of the previous implants have been related mainly to the inadequate restoration of the original mobility and stability of the ankle complex, caused by poor knowledge of the guiding/stabilizing role played by the ligaments involved. The relative contribution of the ligamentous structures and articular surfaces of the joint in its passive and active stability, in fact, had not yet been fully understood.
Since the early Seventies, the disadvantages of ankle arthrodesis have encouraged numerous ankle arthroplasty designs. The designs devised by the pioneers (1970–1979) all featured two-component prostheses, which have been further classified as constrained, semi-constrained and non-constrained. The two-component designs have been also categorized as incongruent (trochlear, bispherical, concave-convex, convex-convex), and congruent (spherical, spheroidal, conical, cylindrical, sliding-cylindrical), according to the shape of the two articular surfaces. The former type enables a better restoration of normal joint motion but poor wear and deformation resistance due to high local stresses resulting from small contact areas and poor inherent stability. The congruent designs can be expected to provide better performance in terms of resistance to wear and surface deformation due to a better pressure distribution, but also an inadequate restoration of the characteristic three-planar rotation and articular gliding. Cylindrical or conical designs can also provide high stability since the surfaces are forced into total conformity under load, restricting motion to a single plane.
Despite the multitude of designs, to this day there are no total ankle arthroplasty designs with clinical results comparable to those achieved with arthrodesis and to those obtained with total hip and total knee replacements. Aseptic loosening of the tibial and/or of the talar components is the most frequent cause of failure, but complications include also deep infection, dehiscence of the surgical wound, lateral and/or medial subluxation of the floating meniscus, lateral talofibular joint impingement, subsidence of the talar component. The relationship between cause of failure and the etiology of the degenerative disease has been studied by many authors with a large variation of the results reported.
There are several reasons for the poor clinical results of specific prior designs, such as excessive bone resection and bearing subluxation. Common problems are antero-posterior and inversion-eversion instability of the unconstrained designs, high contact stresses, high constraint forces of the constrained designs that produce high stress at the interface between the prosthetic component and the bone interface. Considering the different types of prosthesis, the highest failure rates are shown by the constrained designs. Non-constrained designs with incongruent articular surfaces show only slightly better results. However, inherently poor wear and deformation resistance and poor stability of this type of replacement have been reported.
The more recent prosthesis designs feature three components and comprise a floating bearing, introduced to allow full congruence at the articular surfaces in all joint positions in order to minimize wear of the components while coping with the multi-axial nature of the rotation of the ankle. These designs all feature a planar and a curved surface for the floating intermediate element, to allow the characteristic internal/external rotation at the ankle joint. The floating bearing had been introduced to allow a controlled freedom of motion relative to the tibial component, allowing controlled antero-posterior as well as medial-lateral motion (see document U.S. Pat. No. 5,766,259), in such a way as to reduce wear of the surfaces and stress at the interface between the bone and the tibial component of the prosthesis. However, lateral motion is unlikely to occur at the ankle due to the high level of entrapment of the talus within the tibial mortise in the frontal plane. Moreover, no attention has ever been dedicated to understand which joint structures should enable and control this motion.
The most important aim of the present invention is in fact to include the controlling and limiting functions of the ligaments in the design of the ankle prosthesis while minimizing wear.
The limits of prior three-component designs are related to the lack of attention paid to the essential role of the ligaments in restoring the physiological kinematics of the joint. The original pattern of slackening/tightening of the ligaments should be restored to enable the physiological sliding/rolling motion of the articular surfaces, as recently discovered by the inventors. This pattern can be restored only when the shapes of the prosthetic articular surfaces and the geometry of the ligamentous structures retained are compatible, i.e. the articular surfaces move in mutual contact while maintaining some ligament fibers at a constant length. All prior three-component designs have been aimed at replicating, for the talar component, the same radius as measured in intact bones. The introduction of a third component and of a different shape for the tibial mortise surface (of a planar type) would instead have entailed abandoning anatomical criteria for the design and should have led to closer investigation of the perfect compatibility of all passive structures.
The design of an ankle joint prosthesis should aim either to replicate exactly the complete original anatomical geometry of both the ligamentous structures and the articular surfaces, or to restore the original compatible function of the ligaments and of the articular surfaces introducing the floating component, regardless of the original anatomical shapes of the articular surfaces. In the former option, the slight incongruity between the tibial and the talar articular surfaces should be restored, and its lack can be the cause of the failure of cylindrical and spherical designs. In the latter option, instead, any attempt to imitate the anatomical shapes of the articular surfaces characteristic of intact bones should be abandoned. The confusion between these two options (meniscal bearing designs but with articular surfaces approximating anatomically-shaped curves) is thought to be the problem inherent in the three-component prosthesis designs proposed heretofore, which proves the relevance and the originality of the present invention. The three-component meniscal bearing designs of the prior art (U.S. Pat. No. 5,766,259, STAR®) have claimed to have simulated the original articulating surfaces of the ankle joint to enable proper movement of the parts. We believe instead that when a third component, not present in the natural joint, is introduced in the joint and when the natural concave shape of the tibial mortise is replaced with a planar surface, the design of the articular surfaces should be aimed only at restoring the original functions, regardless of the original anatomy of the bones.
Furthermore, all the previous three-component prosthesis designs (see documents U.S. Pat. No. 4,470,158, U.S. Pat. No. 4,755,185, U.S. Pat. No. 5,766,259, U.S. Pat. No. 5,824,106, STAR®) do in fact allow for internal/external rotation (about the longitudinal axis of the tibia) at the interface between the floating bearing and the tibial component, but do not allow ab/adduction (about the antero-posterior anatomical axis), as it occurs in the intact human ankle complex (ankle and subtalar joints) to be replaced. The problems inherent in these designs also relate to the expected poor stability in the transverse plane due to the planar-to-planar interface between two of the components, and lastly to the small, projecting constraint elements used in prior designs to guide the bearing core (pins, incisions and grooves, in documents U.S. Pat. No. 5,824,106 and U.S. Pat. No. 4,755,185).
The proposed aim of restoring the physiological functions of the ligaments during the motion of the replaced joint and in resistance to lesions has been presented only to a limited extent in prior ankle replacement designs. One of the main innovative elements of the present invention is its original introduction of the role of the ligaments in controlling and limiting the movement of the ankle joint complex.